There are a number of known methods of fuel control for staged gas turbine combustion systems. Combustor flame temperature is an ideal parameter to control fuel split between different flow numbers of a combustor fuel injector, particularly when an optimal split is required for “lean burn” conditions. However, flame temperature is too hot to be measured directly. Hence it needs to be synthesised by sensing different engine parameters and making certain assumptions.
U.S. Pat. No. 5,743,079 describes a method for synthesising flame temperature and regulating fuel flow using high pressure compressor (HPC) exit pressure and temperature, fuel flow rate, combustor pressure drop and high pressure turbine (HPT) nozzle guide vane (NGV) capacity. However, this method suffers from susceptibility to dormant sensor faults (e.g. undetected pressure and/or temperature sensor faults), air leakage in the pneumatic pipe from the HPC used by the HPC exit pressure sensing system, inaccurate fuel flow measurement (e.g. large differences between demanded fuel flow to the fuel metering valve and delivered flow from the valve), variation in fuel flow composition (e.g. water contained in the fuel), engine deterioration, undetected engine component faults (e.g. HPT NGV throat damage), inclement weather, surge, engine starting (including windmill relight and auto relight) and engine flame out.
Further, in staging control methods based on synthesising flame temperature, as more engine parameters and assumptions are applied in attempts to improve the robustness and reliability of the synthesis process (particularly for fast transient conditions), the possibility of inaccuracies increases, along with system cost and weight.